A Schottky diode is a unipolar device using electrons as carriers, which is characterized by high switching speed and low forward voltage drop. The limitations of Schottky diodes are the relatively low reverse voltage tolerance and the relatively high reverse leakage current. The limitations are related to the Schottky barrier determined by the metal work function of the metal electrode, the band gap of the intrinsic semiconductor, the type and concentration of dopants in the semiconductor layer, and other factors. For improving the Schottky diode device performance, a trench type Schottky diode was proposed, in which a thermal oxide layer is grown in trenches, and then a polysilicon or metal material is filled in trenches of the device to pinch off the reverse leakage current, so as to reduce the current leakage of the device.
A typical structure of Schottky diode device with MOS trench has been disclosed by U.S. Pat. No. 5,365,102. Please refer to FIGS. 1A-1F illustrating the manufacturing method of the trench MOS barrier Schottky rectifier (TMBSR). As shown in FIG. 1A, a substrate 12 having two opposite surfaces 12a and 12b is provided. The substrate 12 has a relatively heavily-doped cathode region 12c (shown as N+) adjacent to the surface 12a. A relatively lightly-doped drift region 12d (shown as N) preferably extends from the cathode region 12c to the surface 12b. A silicon oxide layer 13 is then grown on the surface 12b to relieve interlayer stress between the substrate 12 and a later-formed silicon nitride layer 15. A photoresist layer 17 is then formed on the silicon nitride layer 15.
In FIG. 1B, a lithography and etching step is performed to partially remove the silicon nitride layer 15, the silicon oxide layer 13 and the substrate 12 so as to form a plurality of discrete mesas 14 in the drift region 12d of the substrate 12 and trench structures 22 with a specific dimension defined by the mesas 14. Referring to FIG. 1C, a thermal oxide layer 16 is formed on the trench sidewalls 22a and the trench bottoms 22b. After removing the remaining portions of the silicon oxide layer 13 and the silicon nitride layer 15, the resultant structure is shown as FIG. 1D. Then, a top metallization step and a backside metallization step are performed to form an anode metal layer 18 on the mesas 14 and a cathode metal layer 20 on the surface 12a (FIGS. 1E and 1F). Accordingly, a Schottky barrier contact is formed on the interface between the semiconductor mesas 14 and the anode metal layer 18. The process of manufacturing the TMBSR is thus completed.
The trench Schottky diode manufactured from the aforementioned process may have a low forward voltage drop. Furthermore, the trench structure can pinch off reverse leakage current, such that the current leakage of the TMBSR would less than that of a Schottky diode without any trench structure. However, stress deriving from trench-etching cannot be effectively released and the Schottky diode may be damaged during the reliability test. In particular, the product including the trench Schottky diode possibly malfunctions because of the small cracks caused by the stress.
Therefore, there is a need of providing an improved trench Schottky diode and associated manufacturing method to overcome the problems encountered in the prior art.